1. Field of the Invention
This invention relates to a wavelength-variable light source apparatus having a wavelength calibration function of emitted light therefrom.
2. Description of the Related Art
Hitherto, normally a semiconductor laser of an external resonator type, which will be hereinafter referred to as LD, has been used as a wavelength-variable light source and an optical filter which is a wavelength selection element has been inserted into an external resonator for providing single-mode oscillation. The wavelength passing through (or reflected on) the optical filter is made mechanically variable, thereby enabling wavelength sweeping in a wide range.
Such a wavelength-variable light source is used to measure the optical characteristics of an optical filter, a communication optical fiber, and the like. Before the measurement conditions are set, it is necessary to calibrate the wavelength of the current light oscillated from the wavelength-variable light source in response to the measurement environment (ambient temperature and the like). For example, a measuring instrument with the wavelength accuracy checked such as a wave meter using a Fabry-Perot interferometer is used to calibrate the wavelength of the wavelength-variable light source.
However, to measure the optical characteristics of an optical filter, a communication optical fiber, and the like, using the conventional wavelength-variable light source, a measuring instrument with the wavelength accuracy checked such as a wave meter needs to be used to calibrate the wavelength of the wavelength-variable light source, thus the wavelength calibration requires a lot of time and labor and an expensive wave meter must be provided, impeding the effective use of the wavelength-variable light source.
Since the wave meter is of large size relative to the main part of the wavelength-variable light source, upsizing and high costs of the wavelength-variable light source result from containing the wave meter in the wavelength-variable light source.
Incidentally, FIG. 11 shows an example of a wavelength-variable light source apparatus using a wavelength-variable light source. A wavelength-variable light source apparatus 101 shown in FIG. 11 is made up of a wavelength-variable light source section 102, a wavelength-variable drive section 103, a drive control section 104, a control section 105, a central processing unit (CPU) 106, a read-only memory (ROM) 107, an LD drive section 108, and a light detection section 109.
The wavelength-variable light source section (TLS) 102 uses an LD of an external resonator type. The mechanical position of an optical filter forming a part of an external resonator is moved by the wavelength-variable drive section 103, whereby the external resonance condition is varied and the wavelength of emitted light can be made variable in a wide range.
The wavelength-variable drive section 103, which is made up of a pulse motor and the like, moves the mechanical position of the optical filter in the wavelength-variable light source section 102 in response to a drive control signal input from the drive control section 104 and outputs a position signal indicating the move position of the optical filter to the drive control section 104 as a rotary encode signal of the pulse motor. The drive control section 104 generates a drive control signal in response to a wavelength-variable control signal input from the control section 105 and outputs the drive control signal to the wavelength-variable drive section 103 and also outputs the position signal (rotary encode signal) input from the wavelength-variable drive section 103 to the control section 105.
The control section 105 has a function of controlling the relationship between the move position of the optical filter in the wavelength-variable light source section 102 and the wavelength of emitted light. The control section 105 generates a wavelength-variable control signal in response to a wavelength-variable instruction signal input from the CPU 106 and outputs the wavelength-variable control signal to the drive control section 104 for setting the wavelength of emitted light. The control section 105 also checks that the emitted light is set to the set wavelength based on the position signal (rotary encode signal) input from the drive control section 104, then stops the drive control. Further, the control section 105 converts the position signal (rotary encode signal) input from the drive control section 104 into position data and outputs the position data to the CPU 106.
The CPU 106 outputs a wavelength-variable instruction signal to the control section 105 for instructing the control section 105 to vary the wavelength of emitted light. The CPU 106 also calculates set wavelength of emitted light based on the position data input from the control section 105 and reads a wavelength correction value to correct the light output level based on the set wavelength from the ROM 107, then supplies the wavelength correction value to the LD drive section 108 for causing the LD drive section 108 to correct the quantity of a drive current supplied from the LD drive section 108 to the wavelength-variable light source section 102.
The ROM 107 stores a wavelength calculation processing program which is executed by the CPU 106 and a wavelength correction table setting a number of wavelength correction values for correcting the output level of emitted light in the wavelength-variable light source section 102 corresponding to the position data input from the control section 105 to the CPU 106.
The LD drive section 108 supplies a drive current to the LD in the wavelength-variable light source section 102 based on a light detection signal input from the light detection section 109 for controlling the output level of emitted light to a constant level. The LD drive section 108 also corrects the drive current so as to correct the light detection characteristic responsive to the wavelength in the light detection section 109 in accordance with the wavelength correction value supplied from the CPU 106 and controls the output level of emitted light to a constant level even if the wavelength of the emitted light from the wavelength-variable light source section 102 is varied.
The light detection section 109 is made up of a lens 109a and a light detection element 109b. The lens 109a emits reference light input from the wavelength-variable light source section 102 via an optical fiber 110 to an optical connection terminal 112 to the light detection element 109b as collimated light. The light detection element 109b receives incident light through the lens 109a and converts the light into a light detection signal having a predetermined voltage level in response to the received light strength with the light reception sensitivity characteristic responsive to the wavelength, then outputs the light detection signal from a detection terminal 113 via a cable 114 to the LD drive section 108.
Thus, in the conventional wavelength-variable light source apparatus 101 shown in FIG. 11, the CPU 106 always monitors the move position in the wavelength-variable drive section 103 and corrects the drive current supplied from the LD drive section 108 to the wavelength-variable light source section 102 in response to the wavelength.
However, in the conventional wavelength-variable light source apparatus 101 shown in FIG. 11, the CPU 106 always monitors the move position in the wavelength-variable drive section 103 and corrects the drive current supplied from the LD drive section 108 to the wavelength-variable light source section 102 in response to the wavelength, thus the processing load on the CPU becomes large and installation of the CPU results in an increases in costs of the wavelength-variable light source apparatus.